The formation of the continental crust and the evolution of Earth’s mantle composition explored utilizing geochemical data and geodynamic models.
Sobolev A.V.1, Sobolev S.V.2,3
1 Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, Grenoble, France.
2 GFZ Helmholtz Center for Geosciences, Potsdam, Germany.
3 University of Potsdam, Institute of Geosciences, Potsdam, Germany.
Keywords: Hadean, Archean, subduction, plate tectonics, mantle plume, trace elements, isotopes, numerical models
Abstract
The main process that changes Earth’s silicate composition after core segregation is the formation and recycling of continental crust. These processes are closely related to the tectonic regimes that operated at different times during Earth’s history. This review combines recent geochemical data and geodynamic models of how continental crust formed throughout Earth’s history, especially during the Hadean and Archean eons. Continental crust cannot form by direct melting of the dry ultramafic mantle. It requires water, mafic protolith, and minerals compatible with high-field-strength elements (Ti, Nb, Ta, Zr, Hf), such as amphibole, rutile, ilmenite, or jadeite pyroxene. For the early Earth, the most likely model involves two stages: first, basaltic or picritic (oceanic) crust is extracted from the mantle, leaving behind a refractory harzburgitic residue. Then, after hydration, the oceanic crust subducts, melts or releases water to flux melting in the mantle, creating continental-crust magmas. Meanwhile, the remaining refractory residue mixes with refractory mantle material, producing a depleted mantle reservoir. Canonical Nb/U and Ce/Pb ratios are unaffected by mantle melting under dry conditions but change during melt generation when amphibole and high-Ti phases are present. Therefore, these ratios are useful indicators of continental crust formation. Geochemical tracers such as (1) Sr isotope compositions of komatiite melts and plagioclase in anorthosites, (2) element ratios in komatiite melts, (3) trace element contents and Hf isotopic compositions of zircon, and (4) decay products of short-lived Sm and Hf isotopes in rocks either support or do not contradict the operation of active continental crust formation and mantle depletion during the Hadean. Production and recycling of continental crust in this period likely involved episodic, short-lived subduction triggered by plumes. Overall, these findings suggest that tectonic regimes in the Hadean (4.4-4.0 Ga after magma-ocean solidification) and in the Eoarchean (4-3.6 Ga) were more dynamic and varied in time and space than previously thought. However, the development of global plate tectonics—requiring a connected network of subduction zones, mid-ocean ridges, and transform faults—could only start later, during the Archean. The causes of the onset of plate tectonics throughout Earth’s history are still debated, and new ideas (such as those involving surface processes, such as the erosion of continents) are being proposed and require further testing.
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